1. Field of the Invention
This invention relates to a method and apparatus for locating the source of an unknown signal received by a plurality of signal relays.
2. Discussion of the Invention
Prior art location systems are known which employ a technique based on determination of the time difference of arrival (TDOA) of two signals from the source to be located, ie an unknown transmitter. In this technique, a signal from the transmitter is relayed to a ground-based receiving station along two independent signal paths by a respective earth-orbiting satellite in each path. Each satellite incorporates a transponder system which accepts a signal (uplink) from a ground transmitter at one frequency and returns a frequency-shifted (downlink) signal to a ground receiver with the aid of a turnround oscillator. The signal paths are of unequal length, because the two satellites are in different positions. In consequence, the signal arrival time at the receiving station associated with one path is different to that associated with the other. The positions of the two satellites and the receiving station are known, and consequently the time difference of arrival--the TDOA--of the signal over the two different paths provides information on the location of the unknown transmitter.
For specific satellite positions, the locus of points of constant TDOA defines a curve on the earth referred to as a line of position (LOP). An unknown ground-based transmitter giving rise to a specific value of TDOA is located at some point on the LOP associated with that value. The geometrical relationship between the satellites together with signal propagation delay in satellite transponders and receiving station equipment, must be known in order to determine the correct value of TDOA.
However, an LOP only locates an unknown transmitter to an arbitrary position on a curve. U.S. Pat. No 5,008,679 discloses a transmitter location system incorporating two relay satellites as previously mentioned, the system employing both the TDOA of signals from an unknown transmitter and what is referred to as their frequency difference of arrival (FDOA). FDOA occurs because the relay satellites are in motion relative to the earth and to one another, and in each case the motion introduces a Doppler shift into the downlink signal frequency. The two satellites' Doppler shifts differ, and this produces a frequency difference or EDOA between signals arriving at the receiving station from respective satellites. As in the TDOA approach, an FDOA measurement defines a line of position (LOP) upon which the unknown transmitter lies. The TDOA and FDOA lines of position generally differ, and their intersection provides the location of the unknown transmitter.
However, the system of U.S. Pat. No. 5,008,679 is subject to a number of constraints. It requires the positions and velocities of the satellites to be known with a high degree of accuracy. It cannot be used effectively with communications signals of 50 kHz bandwidth or less if the satellites have orbital inclinations of more than 0.1 degree relative to the Earth's equatorial plane. The system also requires a very high degree of phase stability from the ground station local oscillators and the satellite signal turnround oscillators. The latter requirement may render the system ineffective with satellites having very low inclination angles, ie less than 0.01 degree; this is because the Doppler shift caused by satellite motion is small and difficult to measure at such inclinations.
Furthermore the system of U.S. Pat. No. 5,008,679 can only be used with collocated receiving ground stations which have common time and frequency reference sources. It cannot be used without modification where the receiving ground stations are geographically separate. The system also requires the use of phase-locked frequency downconversion chains.